Regardless of whether the subordinate clause (i.e. after depuis que ) is still true or not, the present used in the main clause (manger et dormir or être une passionnée) forbids the imperfect to be used in the subordinate clause because the main action is still ongoing:

I have been eating and sleeping since I came back from that trip.

I have been a fan since I was a child.

You only use the imperfect in the subordinate clause when the main action happened in the past:

Je ne faisais que manger et dormir depuis que j’étais revenue de ce voyage. (but this period is over)

Depuis que j’étais petite, j’étais une passionnée. (this might have changed)

As you noticed, both the passé composé or the present are possible in the subordinate.

The passé composé is used when the period is relative while the present is used when the period is absolute, e.g.:

Depuis que j’ai passé mon permis, c’est moi qui conduit. (relative, I passed it one day)

Depuis que j’ai le permis, c’est moi qui conduit. (absolute, I still have it)

Both je suis revenue and je suis petite are telling from when the main clause is ongoing. Using the passé composé with être petite would insist on the fact this is no more true but is a little heavy:

Depuis que j’ai été petite, je suis une passionnée.

The present is used instead as être petite is not a point in time but more a "permanent" state, although I understand how confusing it is for non native speakers.

The present can be used in similar situations where it wouldn’t make sense in English:

Depuis que j’ai quatorze ans, j’aime le rock !: The person talking is no more 14.

On le connaît depuis qu’il marche à quatre pattes: He isn’t toddling anymore.

If the subordinate describes a real point in time, the passé composé is used:

Depuis que j’ai fêté mes quatorze ans, j’aime le rock !

On le connaît depuis qu’il a marché à quatre pattes pour la première fois.

See this Monique L’Huillier’s document that tells the tense mismatch between French and English is an issue for people learning French and that Gabriel Wyler’s page for a description of depuis uses, from which I picked the relationship between passé composé/present and relative/absolute.